In one form of binary continuous inkjet (CIJ) printing, such as described in U.S. Pat. Nos. 6,554,410; 6,588,888; 6,863,385; and 6,866,370, a printhead produces fluid drops by thermal stimulation of the fluid jet of inkjet ink using ring heaters surrounding the nozzle orifice that initiate pinch-off of the fluid ligament and its induced reorganization into a spherical drop during flight. Unlike electrostatic deflection CIJ, the drops are not monodisperse, and two populations of relatively large drops and relatively smaller drops are intentionally produced. Typically, the large drops are employed as printing drops and the small drops are non-printing drops. Additional droplet types undesirably can form, including satellites and coalesced small non-printing drops. In one useful implementation, the volume of a large printing drop is threefold or fourfold that of a normal small non-printing drop; undesired merged non-printing drops (or catch drops) are 2× in volume. Printing and non-printing drops of ink are selected for marking the substrate and return to the ink tank, respectively, by means air deflection steering of the drop stream towards a catcher surface or gutter. Lateral direction of an air stream at the fluid droplet stream in flight imparts orthogonal momentum to the drops that succeeds in driving the non-printing drops to impact the gutter during a critical segment of the flight, but does not drive the printing drops quite so far, and their flight continues until they impact the substrate being printed. Understandably, the air flow must be carefully adjusted to accurately select between the drop populations, and apparatus for providing controlled gas flow is described in the above referenced patents and further, e.g., in U.S. Pat. Nos. 7,682,002 and 7,946,691. If the air flow is insufficient, such that small non-printing drops are not deflected far enough, they can reach the substrate being printed and a marking error of unintended printing occurs that is referred to as “dark defect” (DD), reducing print quality. If the air flow is too aggressive, large printing drops may also be swept into the gutter and not mark the substrate at all, creating another marking error due to the incomplete print image that is referred to as “pick out” (PO). The difference in the air flow settings (e.g., volumetric flow rate, or differential pressure) between the onsets of the two printing defects is referred to as “operating window,” “printing window,” “operating margin” or “printing margin”, or simply “print margin” or “print window.” It is always desirable to enlarge the print window in order to maximize the robustness of the printing process. Operating settings of air flow through positive and negative ducts of the air deflection manifold are typically chosen by printing a test image, and then varying the air flow for each individual jetting module of a line head until it resides within the operating window between the onsets of the two defects. The problem arises, however, that the print margin in a CIJ printing press is both sensitive to the details of the image to be printed and to the printing speed. In particular, when airflows are set to prevent DD, the printing of images containing abrupt changes between regions of continuous white and black, as in a logo, exacerbates the PO defect under high speed printing conditions.
U.S. Pat. No. 7,766,471 B2 to Kato and Tonishi, and U.S. Patent Publication Nos. 2008/0207805 A1 to Blease et al. and 2009/0169761 A1 to Szajewski et al. are directed at drop-on-demand (DOD) thermal bubble or piezoelectric drop ejectors and disclose pigmented aqueous inkjet ink sets where the dynamic surface tension at low surface ages has been reduced by inclusion of surfactants or surface active, polar protic organic co-solvents to address ink-on-ink and other print image quality defects such as intercolor bleed, feathering, mottle and graininess, and coalescence. The DOD ink compositions are typically comprised of a minimum of about 10 wt % of humectant and organic co-solvent to maintain the ink pigment particles in a dispersed state and to prevent the drop ejector from drying out, which would result in crooked or blocked jets. In addition, the stability of the DOD ink compositions to fluidic forces associated with CIJ recirculating filtration is not assured, since the inks are delivered to the nozzles by passive fluidic forces and not active mechanical pumping, and since the fired ink that is ejected from the nozzles is never re-used.